Radio receiving system



P. 0. FARNHAM RADIO RECEIVING SYSTEM l lil'l'll m SCREEN l ya Filed Nov. 4, 1932 PLATE Patented Aug. 28, 1934 Umrso STATES RADIO RECEIVING SYSTEM Paul 0. Farnham, Boonton, N. J., assignor to Radio Frequency Boonton,

Laboratories, Incorporated, N. J a corporation of New Jersey 7 Application November 4, 1932, Serial No. 641,281

10 Claims. (Cl. 250-220) This invention relates to radio receiving systems of the automatic gain control type, and particularly to methods of and circuit arrangement for securing a desirably fiat gain control characteristic in receivers using vacuum tubes with filamentary cathodes.

To minimize the effects of fading and/or to avoid manual changes in sensitivity as the receiver is tuned over the frequency band, it is desirable to reduce the amplifier gain automatically with increases in the radio voltage input but this automatic control should not come into action until the receiver input rises to that critical value which, with maximum amplifier gain, will provide the maximum desired audio output at the'loud speaker. The automatic control of the receiver sensitivity is obtained, in general, by rectifying the output of a radio frequency amplifier to obtain a direct current voltage which is returned to the amplifier as a bias or gain control voltage. More particularly, the preferred types of gain control systems have included rectifier systems in which no rectification took place until the radio amplifier output increased to a certain critical level, but which developed a rectified voltage that increased rapidly with increases in radio output above the critical level. Systems of this type have usually involved the connection of the cathodes of some tubes of the receiver to a point of direct current potential different from that of the cathodes of other tubes.

' Although this condition is easily realized in the case of tubes of the separate heater type, it presents the decided disadvantage that separate sources of filament current supply must be used for the groups of filament type tubes which are to be operated with different direct current potentials on the cathodes.

An object of the present invention is to provide a gain control system for receivers of the filamentary cathode type, which system gives a delayed control action without the use of separate filament supply sources for different groups of tubes. Another object is to provide a radio receiver using filament type tubes, and which includes simple and eiiicient circuit arrangements for delaying a gain control system to obtain a desirable gain radio input characteristic. A further object of the invention is to provide,- inreceivers-of the automatic gain control type, a rectifier system including diode detection. of the modulated radio signal and a delayed or biased diode rectification for the development of a gain control bias. 1 a These and other objects and advantages of the invention will be apparent from the accompanying drawing in which the single figure is a fragmentary circuit diagram of anembodiment of the invention.

In the drawing, the reference numeral 1 identifies a radio frequency amplifier deriving its carrier wave input from an antenna or collector structure 2, and including a plurality of amplifier tubes B'between the antenna and an output coil 4 which is coupled to the tuned input circuit 5 of the detector-automatic control stage. The several tubes of the amplifier 1 and the coupling between the tubes may be of any desired or appropriate type and, in the drawing, only those elements of the tubes which are directly related to the invention have been illustrated.

The detector-control stage preferably includes only a single tube, which may be of the screen grid construction known as type 232, but it will be apparent that the novelmethod of operation may be obtained by the useof separate detector and control tubes.

' One terminal of the tuned'circuit 5 is grounded and the opposite terminal is connected, through condensers 6, 7, 8, respectively, to the inner grid G1, the outer grid G2 and the plate P of the tetrode 9. Each of these cold electrodes cooperates with the filamentary cathode F to function as a diode rectifier. The radio frequency voltage which is impressed between grid G1 and the cathode is rectified in the audio frequency output circuit formed by connecting resistance 10 between the grid: and the grounded or negative terminal of the cathode F. The resulting audio frequency voltage En may be taken off between ground and the terminal 11 which is connected to grid G1. This audio output will usually be further amplified before it reaches the loud speaker or other reproducer, not shown.

The plate P cooperates with the filament or cathode F to form the elements of the diode rectifier of the gain control system. The output resistance 12 is connected between the plate and a tap 13 on the C-battery 14, the direct current potential E1 thus established on the plate being negative with respect to the grounded terminal of filament F. The plate terminal of the output resistance 12 is connected, through a filter comprising resistance 15 and condenser 16, to the lead 17 that extends to theinductances 18, and thence to the grids G of tubes 3.

The low potential terminals of inductances 18 are grounded, for radio frequency potentials, by A condensers 19, and. the lead 17 includes resistances 20 which prevent any coupling of the tube input circuits, through the gain control system.

The filamentary cathodes F of all tubes are connected in parallel across the filament supply or A-battery 21. energizing the plate and screen grid circuits, not shown, of the amplifier tubes. 7

The circuit elements, as so far described, cornprise an automatic gain control system which is operative, but which cannot provide a gain con- A B-battery 22 is provided for trol characteristic having the desired fiat top and a relatively sharp cutoff at the critical value of receiver input. When the tap 13 is initially set to establish a negative bias voltage E1 on the amplifiers 3 which is appropriate for the maximum amplifier gain, the same negative bias is applied to the plate P of the detector-control tube 9. From the known operation-of gain-control systems which include a biased or'delayed action diode rectifier, it is apparent that no rectification will take place untilthe peak radio input voltage E exceeds the delay bias voltage E1. In general, however, an adjustment of voltage E1 for satisfactory amplification in the radio frequency amplifier does not provide a sufiiciently high negative or delay bias on the diode rectifier LPF to obtain a satisfactory control characteristic. On the other hand, an adjustment of the bias voltage E1 to provide a satisfactory delay bias on the rectifier will place such a high negative bias, of the order of from 10 to 20 volts, on the amplifier tubes 3 that the gain of the radio amplifier will be seriously reduced.

In accordance with this invention, the action of the negative or delay bias voltage E1 on the gain control rectifier PF may be effectively increased to prevent rectification until the radio input voltage E on the diode rises to much higher values than that corresponding to the actual bias voltage E1. This effect is obtained by means of "the grid G2 which cooperates with the filament F to form a diode rectifier that is operated under a negative or delay bias applied to the grid G2 through the resistor 23 and the C-battery 14. It has beendemonstrated that as the delay bias voltage E2 is progressively increased to place a greater negative bias on this diode, rectification'in the PF diode circuit does not occur until the radio voltage E reaches correspondingly higher values. The eifectivedelay bias voltage on the gain control rectifier PF may therefore be increased to any appropriate value in excess of the bias E1 which is applied both to the plate of tube 9 and-to the grids of the amplifier tubes 3.

"It is therefore possible to choose an initial bias voltage E1 which is appropriate for maximum gain in the radio amplifier and which, in combination with the delay bias E2 on the'grid G2,

postpones the initiation of the action of the automatic gain control system until the receiver input reaches thecritical value which, with maximum gain, will deliver the desired maximum audio frequency output at the loud speaker. With increasing receiver input voltages above this critical value, the gain control diode passes a direct current voltage which increases at a relatively rapid rate to reduce the amplifier gain and thereby hold the receiver output at'a substantially constant level.

The following valuesof the more important constants of the rectifier circuit were found suitable in one particular case:-

connection with tubes having filamentary cathodes, it will be apparent that the same general method of increasing the effect of a delay'bias anodes, one of said anodes being located between voltage may be employed with tubes of the separate heater type.

I claim:

1. In a radio receiver, the combination with a vacuum tube having a cathode and an anode comprising the elements of a diode having the inherent characteristic of rectifying an impressed alternating current voltage when the peak alternating voltage exceeds that critical value which is the direct current potential between the said diode elements, an input circuit for impressing radio voltages between said elements, and an output circuit, of means preventing rectification by said diode until the peak radio voltage impressed thereon rises to a predetermined value differing from and greater than said critical value by a voltage increment of substantial magnitude.

2. The invention as set forth in claim 1, wherein said rectification preventing means comprises a third tube element, circuit elements cooperating with said third element and cathode to form a second diode rectifier, and bias means for impressing a direct current potential between said third element and cathode.

3. In a radio receiver, the combination with a vacuum tube having a cathode and an anode comprising the elements of a diode rectifier, of an input circuit for impressing radio voltages on said tube, an output circuit, means applying a direct current potential between anode and cathode to bias the diode rectifier to postpone rectification until the peak radio voltage impressed on said rectifier exceeds the applied bias voltage, and means additional to the bias voltage applied between anode and cathode for controlling that value of the impressed radio voltage above which rectification will take place.

4. In a radio receiver, the combination with a vacuum tube having a cathode and at least two said cathode and the other anode, of input circuit means impressingv radio voltages between each anode and the cathode, an output circuit between each anode and the cathode, and delay means preventing the development of a rectified voltage in the output circuit of the outer anode until the radio voltage impressed upon the input circuit thereof exceeds a predetermined value, said delay means including a source of direct current voltage impressing upon the inner anode a bias voltage that is negative with respect to the cathode.

5. In a radio receiver, the combination with a vacuum tube having a cathode and at least two anodes, one of said anodes being located between said cathode and said other anode, each of said anodes together with the cathode comprising the elements of diode rectifiers, of an input circuit for impressing radio voltages on each of said diode rectifiers, an output circuit, and means applying a' direct current potential between the inner anode and cathode, whereby rectification between the cathode and the outer of said anodes is postponed until the impressed radio voltage exceeds a predetermined value. 1 '6. The invention as set forth in claim 3,

wherein said independent means comprises a tube element between the anode and cathode, and circuit elements cooperating with said tube element and cathode to form a diode rectifier, said circuit elements including means applying to the said tube element a direct current potential which is negative with respect to the cathode and different from the negative potential on the anode.

7. In aradio receiver, the oombination'with a radio frequency amplifier, and a rectifier working out of said amplifier, of means for biasing said amplifier to control the gain thereof, said means including said rectifier, a single source of direct current potential for initially applying a negative bias voltage to said amplifier and rectifier, and means preventing rectification by said rectifier until the peak radio input on said rectifier exceeds a value which is substantially greater than that of the negative bias voltage applied to the rectifier.

8. In a radio receiver, a radio frequency amplifier tube having a control grid and cathode, a diode rectifier having an input circuit coupled to the amplifier output circuit, said amplifier tube and diode rectifier having their cathodes connected to each other and maintained at the same direct current potential, a resistance and a source of direct current potential connected between the amplifier cathode and control grid toapply a negative bias to said amplifier tube, a rectifier output circuit including said resistance and said current source, whereby a negative bias is applied to the rectifier anode, and means preventing rectification by said rectifier until the peak radio voltage impressed thereon rises to a critical value substantially greater than that of the negative bias on said rectifier.

9. The invention as set forth in claim 8, wherein said last means comprises a third tube electrode between the rectifier anode and cathode, a diode,

input circuit coupled to the amplifier output circuit and connected between the said electrode and the cathode, and means for applying to said electrode a direct current potential different from that of said anode.

10. In a radio receiver employing filament type tubes, the combination with an amplifier tube having a control grid and filament; an output circuit for said tube; a detector-control stage including a tube having a filamentary cathode, a detector anode, a rectifier anode and a grid between said cathcde and rectifier anode; input circuit means coupled to said amplifier output circuit to impress a radio voltage between said cathode and each of the other tube elements; and an audio output circuit for the cathode-detector anode rectifier; of a common source of current for the amplifier filament and detector-control tube cathode, bias means for the control grid of said amplifier tube and including a direct current source in series with a resistance, a rectifier output circuit including said series current source and resistance, whereby a delay bias is established on said rectifier anode, and a diode output circuit for said grid and cathode, said diode output circuit including a direct current source impressing on the grid-cathode diode a negative bias in excess of the delay bias on said rectifier anode.

PAUL O. FARNHAM. 

